Surprise! It’s someone not connected to Sidney. Karl Gibbs is Sidney and Stab obsessed, and collaborates with others to create a new final girl.
We really don’t know which Ghostface killer is responsible for any of the murders, other than knowing Gibbs is responsible for stabbing Mark Evans.
Ghostface is not immortal but is always hard to kill (hence all the “shoot them in the head” talk). Except Ghostface #1 is mowed over by Gale Weathers’ van, so…
Poor Karl is the first Ghostface to be killed. He’s run over by Gale Weathers’ van (with Gale driving). His murder is a shock in-movie as Ghostface is always someone related to Sidney. Maybe they were shocked, but he seems more like a contrived character than one who contributes anything. He’s there to throw you off.
He’s the first killer in the franchise that has no direct connection to Sidney.